The present invention relates to the field of alarm systems and particularly to an intrusion detection apparatus for use in detecting intrusions into homes, stores, factories and other enclosures.
Prior devices for detecting intrusions have relied upon many different principles. For example, contact switches have been mounted on each door and window of an enclosure. Upon opening the door or window, the switch is actuated to set off an alarm. Similarly, radiation devices have been employed wherein an entry into the enclosure causes the radiation to be interrupted thereby setting off an alarm.
The desire for an improved and more economical intrusion detection apparatus has lead to devices relying upon differential air pressure between an enclosure and the external environment. Such an apparatus is described in U.S. Pat. No. 3,829,851 and in patents cited and discussed therein. These differential pressure devices present a problem in that they must be both sensitive enough to detect significant changes and able to distinguish between intrusion-caused and non-intrusion-caused changes. Non-intrusion-caused changes frequently arise from changes in the external atmosphere such as changes in wind velocity and wind direction. Also flues, vents and normal leaks present problems which must receive compensation. Prior intrusion detection apparatus has not been adequately sensitive to detect changes or has not adequately distinguished between intrusion-caused and non-intrusion-caused changes.
In view of the above background of the invention, it is an object of the present invention to provide an improved differential-pressure intrusion detection apparatus which is highly sensitive to changes while having the ability to distinguish between intrusion-caused and non-intrusion-caused changes.